Apparatus and method for handling easily polymerizable substance, apparatus for extracting liquid from apparatus under reduced pressure, and process for producing easily polymerizable substance

ABSTRACT

In an apparatus capable of distilling and refining an easily polymerizable substance, piping is performed so that when a strainer in action is switched to a backup strainer, the strainer can be switched after removing air in the backup strainer to replace air in the backup strainer with a discharge liquid of a pump. According to the apparatus, switching the strainers installed on an upstream side of the pump for extracting a liquid of a vacuum distillation column, which distills and refines the easily polymerizable substance, can be performed with no trouble during operation of the distillation column. When the apparatus includes a device for measuring a differential pressure between an upstream side and a downstream side of the strainer in action, two or more strainers installed in parallel can be switched at proper times and the easily polymerizable substance can be produced efficiently.

This application is a divisional application of application Ser. No.10/871,577, filed Jun. 21, 2004, now U.S. Pat. No. 7,037,411, which wasa Rule 1.53(b) Continuation of PCT/JP02/13373, filed Dec. 20, 2002.

TECHNICAL FIELD

The present invention relates to an apparatus for extracting a liquid,particularly a liquid potentially containing solids, from an apparatusunder reduced pressure, and to a process for producing an easilypolymerizable substance which comprises switching two or more strainersparallelly installed in a liquid extraction pipe at proper times,wherein the liquid extraction pipe is connected to a pump fortransferring an outlet liquid from a treating column for handling theeasily polymerizable substance.

BACKGROUND ART

When distilling and refining easily polymerizable substances, forexample, acrylic acid and methacrylic acid or their esters, distillationis performed with addition of polymerization inhibitors and underreduced pressure in order to prevent polymerization from occurring.However, it is impossible to completely prevent polymerization and it isinevitable that some polymerization occurs. In particular, for example,a drastic change in conditions under which a distillation apparatus isoperated for any reason may result in an abrupt progress inpolymerization. Therefore, some polymerisate may exist in distillationresidue drawn out from a bottom of the distillation apparatus andpolymerisate may also be contained in condensate in a reflux tank at atop of the apparatus.

Extracting distillation residues at a bottom of the column to anexterior, extracting condensate in the reflux tank at a top of thecolumn to the exterior, or refluxing the condensate to a distillationcolumn needs aid of a pump. However, direct supply of a liquid thatcontains solids such as a polymerisate may cause solids to be attachedto a casing part of the pump or enter into a mechanical seal part of thepump. Once such a phenomenon occurs, flow is stagnated at that portion,so that polymerization of a polymerizable substance in the distillationresidue proceeds, thus causing a serious problem in operation of thepump, and ultimately it becomes inevitable to stop the operation of thepump.

Further, it is possible that solids that passed the pump may causeclogging of a flow meter and control valve provided on a discharge sideof the pump. Therefore, a strainer is provided usually on an upstreamside of the pump in order to avoid feeding of solids to the pump. Toallow for continued operation of the distillation column even when thestrainer is clogged with solids, two or more strainers are provided inparallel. In a case where one strainer which is in operation is clogged,a backup strainer provided in parallel can be immediately brought intooperation by switching valves.

However, in general, switching a strainer of a pump for extracting aliquid from a vacuum distillation apparatus to a backup strainer tendsto cause an abnormal operation of the pump. Also, in a case ofdistillation and refining of a polymerizable substance, switchingstrainers of a pump for extracting a condensate from a reflux tank mayresult in production of a polymerisate in the condensate in the refluxtank. Also in a case of switching the strainers of a pump for extractingdistillation residue from a bottom of a column, production ofpolymerisate in the column may increase.

The inventors of the present invention have studied a cause therefor andas a result, they have found that such is attributable to air containedin the backup strainer. That is, a strainer clogged with solids becomesa backup strainer to be opened and cleaned. These operations areperformed in air and hence atmospheric air is present in the backupstrainer. Accordingly, operation of the backup strainer will result inback flow of inside air to an upstream side to flow into the apparatusunder reduced pressure, or flow into the pump located downstream, orfurther into an apparatus to which a discharge liquid of the pump flows.As a result, operation of the pump is disturbed and the operation of thepump must be stopped, as the case may be.

Further, this disturbed operation of the pump or flowed in air willdisturb flow of steam and liquid within the distillation apparatus, witha result that polymerization of a polymerizable substance is promoted.That is, although a polymerization inhibitor is fed to a condenser atthe top of the column to prevent the condensate from polymerizing whendistilling and refining a polymerizable substance, it is conceivablethat disturbed flow of steam in the condenser will produce a condensatein which concentration of the polymerization inhibitor is locallyreduced, which triggers polymerization.

Further, back flow into the distillation column of the air in the backupstrainer of a pump for extracting distillation residues from the bottomof a column will disturb the flow of the steam and liquid in the column,causing local stagnation of the liquid or local concentration reductionof the polymerization inhibitor to promote polymerization.

In a process where acrylic acid, methacrylic acid, and esters thereof,which are typically easily polymerizable substances, are produced bycatalytic gas phase oxidation, it is important to stabilize operationalconditions of respective unit operation apparatuses used in unitoperations such as separation or purification of acrylic acid,methacrylic acid and esters thereof, from a viewpoint of stableproduction of these substances, which are easily polymerizablesubstances, in order to suppress formation of polymerisate that willfrequently occur during the operation.

Since these substances are easily polymerizable substances, polymerisateis generated inside of a plant in spite of use of a polymerizationinhibitor or a polymerization retardant. For example, polymerisate isproduced inside each distillation column and condenser that constitute aprocess, and the polymerisate is accumulated inside of the distillationcolumn, or a part of or all of the polymerisate falls to the bottom ofthe column, or clogs the condenser, or falls to the reflux tank or thepump connected to the reflux tank through a line.

To remove the polymerisate, for example, a strainer installed on a feedside of a pump for transferring a column bottom liquid that transfersthe column bottom liquid to a downstream appliance is provided forpurposes of protecting the pump, preventing polymerization solids frombeing fed to the appliance downstream of the pump, and the like.Further, since continuous operation is intended, two or more strainersparallelly installed (one strainer being in action and other strainer(s)being in a standby state) are provided.

In a case where an easily polymerizable substance is produced, althoughpolymerisate generated can be collected by the strainer and removed fromthe process thereby, times of switching to the strainer provided as abackup cannot be judged since a state of the polymerisate beingcollected during the operation of the strainer cannot be grasped. Thatis, an increased amount of collection will cause clogging of thestrainer with collected polymerizate, thus generating cavitation in thepump connected to the strainer. This causes abnormality of pumpoperation, which in turn leads to disturbed operational conditions (forexample, variation in pressure, variation in flow rate, and variation intemperature) in appliances installed on upstream and downstream sides ofthe pump, thereby generating polymerisate in appliances to stopoperation of the appliances or the plant in its entirety.

For this reason, the strainers are frequently switched to cope with theaforementioned problems; however, this incurs a labor load to anoperator who operates, and in a case where a substance to be handled isa dangerous substance, toxic substance, or the like, utmost care must betaken and the operator is put under a great psychological burden.

Easily polymerizable substances, particularly acrylic acid, methacrylicacid, and esters thereof have this tendency to such a significantextent. And in an industrial production plant, a countermeasure isdemanded to reduce a burden on the operator and avoid a situation wherethe operation has to be stopped.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an apparatus forextracting a liquid, and an apparatus and method for handling an easilypolymerizable substance, in which problems of operation caused by aircontained in a backup strainer can be avoided.

Further, another object of the present invention is to switch two ormore strainers at proper times, wherein the strainers are parallellyinstalled in a liquid extraction pipe for transferring an outlet liquidof a treating column from the treating column for handling the easilypolymerizable substance to the pump for transferring the outlet liquid,in a process for producing an easily polymerizable substance and anapparatus and method for handling an easily polymerizable substance.

The apparatus and method for handling an easily polymerizable substanceaccording to the present invention is as follows.

(1) An apparatus for handling an easily polymerizable substance,comprising: a decompressor for handling the easily polymerizablesubstance under reduced pressure; a liquid extraction pipe having anupstream end that opens in the decompressor; a pump provided in a middleof the liquid extraction pipe; a plurality of strainers connectedparallel to one another to the liquid extraction pipe on an upstreamside of the pump; and valves provided on an upstream side and downstreamside of each strainer, wherein the apparatus comprises at least one ofconstructions (A) and (B) below:

(A) an exhaust device for exhausting air in a part sandwiched betweenthe valves provided on the upstream side and downstream side of eachstrainer, and a branch pipe that connects the part sandwiched betweenthe valves provided on the upstream side and downstream side of eachstrainer and the liquid extraction pipe on the downstream side of thepump; and

(B) a device for measuring a differential pressure between a pressure inthe liquid extraction pipe on the upstream side of each strainer and apressure in the liquid extraction pipe on the downstream side of eachstrainer.

(2) The apparatus according to (1), wherein the exhaust device isconfigured such that when a liquid is introduced from the branch pipe tothe part sandwiched between the valves provided on the upstream side anddownstream side of each strainer, the liquid expels air in the part.

(3) The apparatus according to (1) or (2), wherein the decompressor forhandling the easily polymerizable substance under reduced pressure is anapparatus for distilling an easily polymerizable polymer selected fromthe group consisting of acrylic acid, methacrylic acid, and estersthereof under reduced pressure.

(4) A method for handling an easily polymerizable substance using theapparatus according to any one of (1) to (3), comprising: using thestrainers one by one; determining a time for switching the strainers tomeasure a difference between a pressure in the liquid extraction pipe onan upstream side of the strainer being used and a pressure in the liquidextraction pipe on a downstream side of the strainer being used; andswitching the strainer being used to another one of the strainers.

(5) The method according to (4), further comprising: discharging air ina part sandwiched between valves provided on the upstream side anddownstream side of the another strainer; feeding a discharge liquid ofthe pump from the branch pipe to the part; and switching the strainerbeing used to another strainer after replacing air in the part withdischarge liquid.

The apparatus for extracting a liquid from an apparatus under reducedpressure according to the present invention is as follows.

(6) An apparatus for extracting a liquid from an apparatus under reducedpressure, comprising: a liquid extraction pipe for transferring a liquidunder reduced pressure contained in an apparatus under reduced pressure,having an upstream end that opens in the apparatus under reducedpressure, and having a pump in a middle of the pipe; strainers connectedin parallel to the liquid extraction pipe on an upstream side of thepump; valves provided on an upstream side and downstream side of eachstrainer; an exhaust device for exhausting air in a part sandwichedbetween the valves; and a branch pipe that connects a part sandwichedbetween the valves provided on the upstream side and downstream side ofeach strainer and the liquid extraction pipe on a downstream of thepump.

(7) The apparatus according to (6), wherein the exhaust device isconfigured such that when a liquid is introduced from the branch pipe tothe part sandwiched between the valves provided on the upstream side anddownstream side of each strainer, the liquid expels air from the part.

(8) The apparatus according to (6) or (7), wherein the apparatus underreduced pressure is a vacuum distillation apparatus for an easilypolymerizable substance selected from the group consisting of acrylicacid, methacrylic acid, and esters thereof.

In the apparatus for extracting a liquid from an apparatus under reducedpressure according to the present invention, switching to a backupstrainer is performed as follows. First, discharged liquid from the pumpis introduced to the backup strainer through a branch pipe by switchingvalves, air in the backup strainer is replaced by the discharged liquid,and then the backup strainer is brought into action. Therefore, adisturbed operation due to air in the backup strainer can be avoided.

Further, the inventors of the present invention have made diligent studyon the aforementioned problem, and as a result they have found thatproper times for switching strainers can be determined by measuring adifferential pressure between a pressure in the liquid extraction pipeon the upstream side of the strainer being used and a pressure in theliquid extraction pipe on the downstream side of the strainer being usedin order to grasp a condition of the strainer. Thus, the presentinvention is accomplished.

That is, a process for producing an easily polymerizable substanceaccording to the present invention is as follows.

(9) A process for producing an easily polymerizable substance using atreating apparatus having one or two, or more, treating columns fortreating the easily polymerizable substance, a pump provided in eachtreating column for transferring an outlet liquid of the treatingcolumn, and two or more strainers installed parallelly in a liquidextraction pipe connecting the treating column and the pump, wherein theprocess comprises:

-   -   using the two or more strainers one by one in each treating        column; determining a time of switching the strainers to measure        a difference between a pressure in the liquid extraction pipe on        an upstream side of the strainer being used and a pressure in        the liquid extraction pipe on a downstream side of the strainer        being used; and switching the strainer being used to another        strainer out of the two or more strainers.

(10) The process according to (9), wherein a differential pressure ofthe strainer being used is continuously measured.

(11) The process according to (9), wherein extraction nozzles forconnecting a detection end for measuring pressure are installed to a topof the liquid extraction pipe of the strainer being used.

(12) The process according to any one of (9) to (11), in which theeasily polymerizable substance is selected from the group consisting ofacrylic acid, methacrylic acid, and esters thereof.

(13) The process according to any one of (9) to (12), wherein thetreating apparatus comprises the treating column(s) selected from anyone of a distillation column, an evaporation column, a reflux tank, anextraction column, an absorption column, a high boiling pointdecomposition reactor, an esterification reactor, a heat exchanger orcombinations thereof.

<An Apparatus of the Present Invention for Extracting a Liquid from anApparatus Under Reduced Pressure>

The present invention will be described in more detail with reference tothe attached drawings. FIG. 1 shows an example of an apparatus forextracting a liquid according to the present invention.

Reference numerals 1 and 2 stand for strainers. The strainers may bethose employed in general chemical apparatus. For example, Y typestrainers, bucket type strainers, and the like are used.

Reference numeral 3 indicates a pump. Similarly to the strainers, abackup pump may be provided parallelly as the case may be. Referencenumeral 4 indicates an upstream side liquid extracting pipe, 5 indicatesa downstream side liquid extracting pipe, and 6 is a branch pipe.

The upstream side liquid extracting pipe 4 is a pipe for transferring aliquid under reduced pressure contained in an apparatus under reducedpressure, and has an upstream end opening in the apparatus under reducedpressure though not shown. In the present invention, “upstream endopening in the apparatus under reduced pressure” means that the upstreamend of the liquid extracting pipe opens in a chamber, column and thelike in which liquid under reduced pressure is contained. The apparatusunder reduced pressure includes, for example, a vacuum distillationapparatus, specifically the apparatus shown in FIG. 2 or FIG. 4.Substances handled in such an apparatus include, for example, easilypolymerizable substances selected from the group consisting of acrylicacid, methacrylic acid, and esters thereof.

A valve a and a valve b are provided upstream and downstream of thestrainer 1, respectively, and the strainer 1 is also provided with anexhaust valve h. Also, a valve c and a valve d are provided upstream anddownstream of the strainer 2, respectively, and the strainer 2 is alsoprovided with an exhaust valve i.

The exhaust valves h and i are used for expelling and discharging air ina part sandwiched between valves a and b, or c and d, by liquiddischarged from the pump 3 and fed through the branch pipe 6; therefore,the exhaust valves h and i are provided at a highest position betweenthe part sandwiched between the valves a and b, or c and d.

Note that in FIG. 1, the branch pipe 6 opens between the strainer 1 andthe valve a. However, it may open between the strainer 1 and the valveb. The same is true for the strainer 2; the branch pipe 6 may be openedon a downstream side of the strainer 2.

Action of the liquid extraction apparatus is described as follows. Whenthe strainer 1 is in action, all the valves are closed except for thevalves a and b that are open. Upon switching from the strainer 1 to thestrainer 2, valves i and g are opened, and then valve e is slowly openedto allow the liquid discharged from the pump to flow into the strainer 2through the branch pipe 6. Inclusive of the strainer 2, air in the partsandwiched between valves c and d is discharged form the exhaust valve iby being pushed by a discharge liquid.

When the part sandwiched between valves c and d is completely replacedby the discharge liquid, valves e, g and i are closed, and valves c andd are opened to bring the strainer 2 into action. Then, valves a and bare closed and the strainer 1 is separated from the line. Subsequently,valves j and h are opened to draw out liquid from within the strainer 1.Thereafter, the strainer 1 is opened and an interior thereof is clearedto obtain a backup strainer. It is preferable that liquid inside thebranch pipe 6 is drawn out by opening valve k.

<A Process for Producing an Easily Polymerizable Substance According tothe Present Invention>

Hereinafter, a process for producing an easily polymerizable substanceaccording to the present invention will be described in detail.

The present invention is a process for producing an easily polymerizablesubstance using a treating apparatus including: one or two, or more,treating columns for handling an easily polymerizable substance; a pumpprovided for each treating column for transferring an outlet liquid fromthe treating column; and two or more strainers parallelly installed in aliquid extraction pipe extending from the treating column to the pump,wherein: in each treating column, one each of the two or more strainersis used, a time for switching the strainers is determined by measuring adifference between pressure on an upstream side of one strainer of theliquid extraction pipe used on the strainer side, and pressure on adownstream side of the strainer of the liquid extraction pipe, and theone strainer is switched to another strainer out of the two or morestrainers.

Examples of typical easily polymerizable substances include acrylicacid, methacrylic acid, and esters thereof. Examples of the acrylatesinclude methyl acrylate, ethyl acrylate, butyl acrylate, isobutylacrylate, tertiarybutyl acrylate, 2-ethylhexyl acrylate, 2-hydroxyethylacrylate, 2-hydroxypropyl acrylate, and methoxtethyl acrylate.

Examples of the methacrylates include methyl methacrylate, butylmethacrylate, isobutyl methacrylate, tertiarybutyl methacrylate, and2-hydroxyethyl methacrylate.

In the treating apparatus used in the present invention, the treatingcolumn for handling an easily polymerizable substance includes any oneof, or combinations of, a distillation column, an evaporation column, areflux tank, an extraction column, an absorption column, a high boilingpoint decomposition reactor, an esterification reactor, and a heatexchanger. In a case of combinations of two or more of these components,each apparatus that constitutes the treating column is provided with apump for transferring an outlet liquid, and two or more strainersparallelly installed in a liquid extraction pipe of the pump extendingfrom the treating column.

The pump employed in the present invention is used for transferring anoutlet liquid from the treating column to a subsequent appliance, andspecific examples thereof include a general reciprocating pump, a rotarypump, a volute pump, an axial flow pump, and a mixed flow pump as wellas a specialty pump such as a jet pump and the like.

In addition, the subsequent appliance includes the aforementionedtreating columns for handling easily polymerizable substances in thepresent invention, such as any one of, or combinations of, adistillation column, an evaporation column, a reflux tank, an extractioncolumn, an absorption column, a high boiling point decompositionreactor, an esterification reactor, and a heat exchanger as well astanks, waste water disposing appliance, incinerators and the like.

FIG. 2 shows one example of an appliance that constitutes the treatingapparatus for handling the easily polymerizable substances.Specifically, the treating apparatus including a distillation column forproducing acrylic acid, methacrylic acid, and esters thereof as atreating column is shown.

The treating apparatus shown in FIG. 2 includes distillation columnsinstalled in series or parallel, a pump for transferring an outletliquid from each distillation column, and two or more strainersparallelly installed in a liquid extraction pipe connecting eachdistillation column and the pump (provided that in FIG. 2, onedistillation column and one pump are illustrated).

In some cases, an evaporation column, an extraction column and the likemay be installed as necessary between the distillation columns, and eachof these is installed with a pump for transferring outlet liquid and twoor more strainers parallelly installed in the liquid extraction pipeconnecting the distillation column and the pump.

Liquid fed from an introduction line 7 to a distillation column 8 isdistilled. Distilled gas is transferred from an outlet at a top of thecolumn to a condenser 9 where the gas is cooled. Then a liquid istransferred to a reflux tank 10. A portion of the liquid in the refluxtank 10 is returned to the distillation column 8 via a liquid extractionpipe 11 for transferring outlet liquid from the reflux tank 10 and pump12 through a reflux line 13. A remainder is extracted from a column topextraction line 14 and transferred to a subsequent appliance.

Spent gas is transferred to a spent condenser 16 through a bent line 15to be cooled, and after valuable substances in the gas are recovered,the spent gas is introduced into a vacuum appliance 17 as the case maybe. Depending on operational conditions of the treating apparatus, thevacuum appliance 17 is unnecessary.

Here, the valuable substances indicate organic substances handled in thedistilling column 8. For example, when manufacturing acrylic acid,valuable substances include acrylic acid, the solvents used whenmanufacturing acrylic acid (such as methyl ethyl ketone, methyl isobutylketone, toluene, and isopropyl acetate). When manufacturing acrylates,the valuable substances include methyl acrylate, ethyl acrylate, butylacrylate, isobutyl acrylate, tertiarybutyl acrylate, 2-ethylhexylacrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, andmethoxyethyl acrylate and the like; methanol, ethanol, 2-ethylhexanol,methoxyethanol and the like as raw materials; and benzene, toluene andthe like as a solvent.

A condensate and non-condensed gas are discharged from the vacuumappliance 17 through an exhaust line 18. Since a small amount ofvaluable substances are contained in the exhaust line 18, the condensateand the non-condensed gas are recycled to a production process foracrylic acid, methacrylic acid, and esters thereof, as the case may be.

To transfer a column bottom liquid (outlet liquid) of the distillationcolumn 8 to a subsequent appliance, the outlet liquid is extractedthrough a liquid extraction pipe 19, pump 20 and column bottomextraction line 21. A portion of the column bottom liquid is returned tothe distillation column 8 via a reboiler 23 through a reboilercirculation line 22.

Since FIG. 2 is a schematic drawing, a polymerization inhibitor liquidextraction pipe, and an air liquid extraction pipe that are necessarydepending on conditions of the system in distillation of acrylic acid,methacrylic acid, and esters thereof are omitted.

Referring to FIG. 3, explanation will be made of a monitoring system forconditions of two or more strainers parallelly installed in a liquidextraction pipe extending from a pump for transferring outlet liquid ofa treating column from the treating column to a subsequent appliance.

The aforementioned strainers are parallelly installed for the treatingcolumns and corresponding pumps in a number of two or more for one pumpin the liquid extraction pipe. During operation, the strainers are usedone by one. The present invention is characterized by determining a timeof switching the strainers to measure a difference between pressure onthe upstream side of a strainer being used of the liquid extraction pipeand pressure on a downstream side of the strainer being used of theliquid extraction pipe, and switching the strainer to another strainer.

The number of strainers installed for one pump is not particularlylimited; usually three or less, preferably two. The number of strainersbeing four or more is not cost effective since the number of strainersthat do not participate in an operation is increased. Note that a sizeof the strainers may be the same or different.

Liquid is fed from a treating column (in FIG. 2, the distillation column8, reflux tank 10, and the like) via a liquid extraction pipe 24 to thepump 3. In FIG. 3, two strainers, the strainers 1 and 2, are installedparallelly with respect to the pump 3 and the strainer 1 is used and thestrainer 2 is a backup with valves a, b, 1, and o being open and othervalves being closed.

A difference between pressure of liquid extraction pipe 24, that ispressure of an upstream side of the strainer (a pressure detection part25 of the liquid extraction pipe), and pressure of the liquid extractionpipe 26, that is the pressure of the downstream side of the strainer (apressure detection part 27 of the liquid extraction pipe) is observed ona differential pressure gauge 28. This observed information istransmitted to an instrument room installed in the treating apparatus.In a case where entrapment of polymerisate is in small amounts, it issufficient that this differential pressure can be observed in situ. InFIG. 2, as a cable for transmitting signals from a detection terminal asobserved information, pressure information transmission lines 29 and 30are installed.

As polymerisate is accumulated in the strainer 1, the difference betweenthe pressure of the liquid extraction pipe 24 (the pressure detectionpart 25 of the liquid extraction pipe) and the pressure of the liquidextraction pipe 26 (the pressure detection part 27 of the liquidextraction pipe) is increased. Judgment of a time of switching when thedifference comes to be increased depends on a condition of the system(capability of the pump, composition of liquid in each line, diameter ofthe pipe in each line, and the like) and hence is not limited.Generally, 1.5 to 150 times, preferably 1.5 to 100 times, morepreferably 1.5 to 50 times based on the differential pressure in a statewhere no polymerisate is present (initial stage of operation of thestrainer).

The strainers used in the treating apparatus in the present inventionare not particularly limited in terms of their type; any one of thoseused in general chemical plants may be used. Specific examples thereofinclude Y type strainers, bucket type strainers, T type strainers, andthe like.

A format and a differential pressure gauge for detection of pressuresfor measuring a differential pressure of the strainers in the presentinvention are not particularly limited, and any of those used in generalchemical plants may be employed. Specifically, examples of a pressuredetector include a diaphragm type pressure gauge. Examples of thedifferential pressure gauge include a diaphragm displacement typeelectronic pressure difference transmitter (generally referred to as acapillary type). For example, EDR-7 manufactured by Hitachi Ltd.corresponds to this.

A position of installing extraction nozzles (31 and 32 in FIG. 3) forconnecting a pressure detection end to a liquid extraction pipe formeasuring a differential pressure of the strainer in the presentinvention is preferably in an upper part of the liquid extraction pipeof the strainer being used (liquid extraction pipes 24 and 26 in FIG.3). Although detection is possible even when the nozzles are installedat the same horizontal position as the liquid extraction pipes 24 and26, or in a lower part of the liquid extraction pipes, influence ofliquid flow in the liquid extraction pipe is likely, so that installingin an upper part of the liquid extraction pipes is optimal. An angle ofinstallment of the nozzles with respect to the liquid extraction pipe ispreferably 5 to 175°, more preferably 70 to 110°, in the upper part ofthe liquid extraction pipes. An optimal angle is 90°.

Generally, extraction nozzles for connecting the pressure detection endand the liquid extraction pipe are installed. A length of eachextraction nozzle is within 800 mm, and within preferably 500 mm interms of horizontal piping. A long extraction nozzle may generatepolymerisate due to stagnation of liquid. The extraction nozzle thatconnects the liquid extraction pipe and the pressure detection end maybe installed with a valve which can open and close. An infimum ofdistance is not limited since the pressure detector may be installeddirectly in the liquid extraction pipe depending on a kind of thepressure detector.

<An Apparatus and a Method for Handling an Easily PolymerizableSubstance of the Present Invention>

An apparatus for handling an easily polymerizable substance of thepresent invention comprises: a decompressor for handling the easilypolymerizable substance under reduced pressure; a liquid extraction pipehaving an upstream end that opens in the decompressor; a pump providedin a middle of the liquid extraction pipe; a plurality of strainersconnected parallel to one another, to the liquid extraction pipe on anupstream side of the pump; and valves provided on an upstream side anddownstream side of each of the strainers, and comprises at least one ofconstructions (A) and (B) below:

-   -   (A) an exhaust device for exhausting air in a part sandwiched        between the valves provided on the upstream side and downstream        side of each strainer, and a branch pipe that connects the part        sandwiched between the valves provided on the upstream side and        downstream side of each strainer and the liquid extraction pipe        on the downstream side of the pump; and    -   (B) a device for measuring a differential pressure between a        pressure in the liquid extraction pipe on the upstream side of        each strainer and a pressure in the liquid extraction pipe on        the downstream side of each strainer.

The apparatus for handling the easily polymerizable substance of thepresent invention may have only construction (A) or only construction(B) out of the aforementioned constructions (A) and (B), or bothconstructions (A) and (B).

In a case where the apparatus has only construction (A) out ofconstructions (A) and (B), the apparatus for handling the easilypolymerizable substance of the present invention can avoid occurrence ofdisturbance in operation caused by air contained in a backup strainer.

In a case where the apparatus has only construction (B) out ofconstructions (A) and (B), the apparatus for handling the easilypolymerizable substance of the present invention can switch the two ormore strainers parallelly installed in the liquid extraction pipeconnecting the pump for transferring an outlet liquid of the treatingcolumn and the treating column for handling the easily polymerizablesubstance at proper times.

In a case where the apparatus has both constructions (A) and (B) out ofthe above-mentioned constructions (A) and (B), it becomes possible toavoid problems of operation caused by air contained in a backupstrainer, and to switch two or more strainers parallelly installed inthe liquid extracting pipe for transferring the outlet liquid of thetreating column from the treating column for handling an easilypolymerizable substance to the pump for transferring the outlet liquidof the treating column at proper times.

The aforementioned decompressor may be constructed by an apparatus forhandling an easily polymerizable substance and an apparatus for formingan atmosphere under reduced pressure. As such a decompressor, forexample, an apparatus for distilling an easily polymerizable substanceselected from the group consisting of acrylic acid, methacrylic acid,and esters thereof under reduced pressure may be mentioned. As theapparatus for handling the easily polymerizable substance may be usedthe aforementioned treating apparatus. As the apparatus for forming anatmosphere under reduced pressure, a well-known vacuum appliance such asa vacuum pump may be used.

Note that construction of the aforementioned decompressor is not limitedto one that has an apparatus for forming atmosphere under reducedpressure, but may be an apparatus for handling an easily polymerizablesubstance in an atmosphere under reduced pressure. Such apparatusincludes, for example, the aforementioned treating column such as a heatexchanger, and the like, connected to a vacuum pump.

An outlet liquid of the aforementioned decompressor is a liquidextracted from the decompressor. Examples of such a liquid includecolumn bottom liquid of the distillation column and condensate of a gascomponent obtained by distillation and so forth.

The upstream end of the aforementioned liquid extraction pipe is open inthe decompressor. This means that the upstream end of the liquidextraction pipe is open in a chamber or a column formed so as to beunder a reduced pressure atmosphere.

The aforementioned exhaust device may be a device that forciblydischarges air in the system such as a vacuum appliance, e.g., a vacuumpump. It is preferable from a view point of simplification ofconstruction of the apparatus to configure, that a liquid introducedfrom the aforementioned branch pipe to the part sandwiched between thevalves installed on an upstream side and downstream side of eachstrainer will expel air in this part.

For the aforementioned device for measuring the differential pressure, awell-known device that can measure pressure of a liquid in a pipe may beused similarly to a differential pressure gauge used in the treatingapparatus employed in the method for producing the easily polymerizablesubstance of the present invention.

Further, the aforementioned device for measuring differential pressureis preferably one that is installed by using the aforementionedextraction nozzles. Also, in the present invention, the aforementioneddevice for measuring the differential pressure is preferably providedsuch that the differential pressure measured can be transmitted to aninstrument room, for the aforementioned decompressor, for continuouslymeasuring the differential pressure of the aforementioned strainers.

The apparatus for handling an easily polymerizable substance of thepresent invention may be specifically configured by applying theapparatus shown in FIG. 1 to the apparatus shown in FIG. 2 or FIG. 4, orapplying the apparatus shown in FIG. 3 to the apparatus shown in FIG. 2or FIG. 4.

A method for handling an easily polymerizable substance of the presentinvention is a method for handling an easily polymerizable substanceusing the apparatus for handling an easily polymerizable substance ofthe present invention, and comprises: using a plurality of strainers oneby one; determining times for switching the strainers to measure adifference between a pressure in a liquid extraction pipe on an upstreamside of the strainer being used and a pressure in the liquid extractionpipe on a downstream side of the strainer being used; and switching thestrainer being used to another strainer out of the plurality ofstrainers.

According to this method, two or more strainers parallelly installed inthe liquid extraction pipe connecting the pump, for transferring anoutlet liquid of a treating column and the treating column for handlingthe easily polymerizable substance, can be switched at proper times.

Further, when the method for handling an easily polymerizable substanceof the present invention further includes: discharging air in a partsandwiched between valves provided on an upstream side and downstreamside of the another strainer; feeding a discharge liquid of the pumpfrom the branch pipe to the part, and switching the strainer being usedto another strainer after replacing air in the part with the dischargeliquid, it also becomes possible to avoid problems of operation causedby air contained in a backup strainer.

Note that, in the invention, “handling” an easily polymerizablesubstance means various operations related to a liquid containing aneasily polymerizable substance such as production of an easilypolymerizable substance, separation of an easily polymerizablesubstance, washing of an easily polymerizable substance, purification ofan easily polymerizable substance, and transportation of an easilypolymerizable substance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing one example of an apparatus for extracting aliquid according to the present invention.

FIG. 2 is a diagram showing one embodiment of a treating apparatus usedin a process for producing an easily polymerizable substance of thepresent invention.

FIG. 3 is a schematic diagram showing a pump and its surroundings of atreating apparatus used in the present invention.

FIG. 4 shows one example of a vacuum distillation column to which anapparatus for extracting a liquid according to the present invention isapplied.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described in more detail byexamples and comparative examples. However, the present invention is notlimited to the following examples unless going beyond the gist of thepresent invention.

EXAMPLE 1

Explanation will be made of a case where the liquid extraction apparatusaccording to the present invention was applied to the vacuumdistillation apparatus shown in FIG. 4, and purifies acrylic acid toform high purity acrylic acid.

In FIG. 4, reference numeral 8 stands for a distillation column, 23stands for a reboiler, 33 stands for a reboiler feed pump, 10 stands fora reflux tank, 12 stands for a pump for extracting a condensate, 20stands for a pump for extracting a column bottom liquid, and 9 and 16stand for condensers. At a top and bottom of the column 8 are attachedthe liquid extraction apparatus shown in FIG. 1, each having a pump 12and a pump 20. Both strainers 1 and 2 are basket type strainers. Notethat in FIG. 4, a polymerization inhibitor feeding apparatus, a vacuumgenerating appliance, and other ancillary appliance are omitted.

The aforementioned vacuum distillation apparatus was operated with thedistillation column being at a column top pressure of 3 kPa and a columnbottom pressure of 11 kPa. During this operation, switching from thestrainer 1 to the strainer 2 at the column top and the column bottom ofthe liquid extraction apparatus performed in the aforementioned methodcaused neither change in an operation of the pump nor change indifferential pressure between the column bottom and the column top.

After 3 months, the strainer 2 of the liquid extraction apparatus at thecolumn top and the column bottom was switched to the strainer 1 again inthe aforementioned method. Checking an interior of the strainer 2indicated no polymerisate.

COMPARATIVE EXAMPLE 1

In contrast, switching from strainer 1 to strainer 2 of the column topand the column bottom of the liquid extraction apparatus was performedby a method in which valves c and d were slowly opened while othervalves remained closed. Then, both pumps generated abnormal sounds andpressure of the distillation column abruptly varied. This is consideredbecause air in the part sandwiched by valves c and d including thestrainer 2 flowed into the distillation column via the pump, or flowedbackward.

After a while, the abnormal sounds of the pump disappeared and valves aand b were closed, thus completing switching of the strainers.Subsequently, continued operation of the distillation column led to agradual increase in differential pressure between the column bottom andthe column top.

Then, the strainer 2 of the liquid extraction apparatus at the columntop and the column bottom was switched again to the strainer 1. Checkingan interior of each strainer 2 indicated polymerisate.

EXAMPLE 2

By using the treating apparatus with the distillation column 8 as shownin FIG. 2, acrylic acid was purified.

The distillation column 8 had a column top pressure at 3 kPa and acolumn bottom pressure of 11 kPa, and a differential pressure ofstrainers installed in a liquid extraction pipe of a pump fortransferring an outlet liquid from the column bottom was continuouslymeasured. Note that a condition of the surroundings of the pump was thesame as that shown in FIG. 3 except that in this example, two backupstrainers were provided. Extraction nozzles (31 and 32) for connecting apressure detection end of a differential pressure gauge to liquidextraction pipes 24 and 26 were installed at 90° to the liquidextraction pipe and had a length of 150 mm.

A differential pressure at an initial stage of operation of the strainerwas 1.6 kPa. After continuous operation, the differential pressure wasincreased slowly and reached 19 kPa after 35 days. Then, the strainerwas switched and an interior thereof was checked. Then, polymerisate wasconfirmed.

Operation of the distillation column was continued, and after 30 days,the differential pressure became 20 kPa so that the strainer was furtherswitched and an interior thereof was checked. Polymerisate was confirmedin this strainer. Operation of the distillation column could becontinued further.

As described above, a time of switching the strainers could be welljudged.

COMPARATIVE EXAMPLE 2

By using the same treating apparatus as that in Example 2, except thatthe apparatus had no differential pressure gauge, as the treatingapparatus, an operation was performed. After 42 days, the pump generatedcavitation so that a strainer was switched urgently. An interior of thestrainer was full of polymerisate.

Cavitation resulted in malfunction of the operation of the distillationcolumn, and after 3 days, cavitation of the pump occurred again. Aninterior of the strainer was full of polymerisate.

Again, an operation was performed after switching the strainer. The nextday, cavitation of the pump occurred again.

Thereafter, an operation with switching of the strainer after occurrenceof cavitation resulted in a shorter interval of occurrence ofcavitation, and finally the operation became impossible.

The operation of the distillation column was stopped and an interior ofthe distillation column was checked. As a result, a large amount ofpolymerisate was confirmed.

INDUSTRIAL APPLICABILITY

Use of an apparatus for extracting a liquid according to the presentinvention enables extraction of distillation residue at a column bottomand a condensate from a reflux tank at a column top from, for example, avacuum distillation apparatus for a polymerizable substance without anyproblems caused to operation of the vacuum distillation apparatus.

Further, according to the present invention, by measuring a differentialpressure between pressure on an upstream side of a strainer and pressureon a downstream side of the strainer installed in a top of a liquidextraction pipe connecting a treating column, used for the treatingapparatus for handling an easily polymerizable substance, and a pump,and determining times of switching strainers, cavitation of the pumpduring operation can be avoided, thereby preventing variation inoperation of the pump or an appliance connected before and after thepump, and providing a process for efficiently producing an easilypolymerizable substance.

Further, according to the apparatus and the method for handling aneasily polymerizable substance of the present invention, in handling ofthe easily polymerizable substance, problems of operation caused by aircontained in a backup strainer can be avoided; two or more strainersparallelly installed in a liquid extracting pipe for transferring anoutlet liquid of the treating column from a treating column for handlingan easily polymerizable substance to a pump for transferring the outletliquid of the treating column can be switched at proper times; or theaforementioned problems in the operation can be avoided and theaforementioned strainers can be switched at proper times.

1. A method for handling an easily polymerizable substance, comprising:feeding liquid containing an easily polymerizable substance from adecompressor, used to handle said liquid under reduced pressure, via afeed pipe, having an upstream end that opens in said decompressor,through one of a first strainer and a second strainer, with said firstand second strainers being connected in parallel with one another tosaid feed pipe; measuring a difference between a first pressure in saidfeed pipe on an upstream side of said one of said first and secondstrainers and a second pressure in said feed pipe on a downstream sideof said one of said first and second strainers; and based on thedifference between said first and second pressures, as measured, (i)stopping the feeding of said liquid through said one of said first andsecond strainers, and (ii) initiating feeding of said liquid, via saidfeed pipe, through the other of said first and second strainers, whereinassociated with the other of said first and second strainers is (i) afirst valve on an upstream side of the other of said first and secondstrainers, (ii) a second valve on a downstream side of the other of saidfirst and second strainers, and (iii) an exhaust device, whereininitiating feeding of said liquid through the other of said first andsecond strainers comprises feeding said liquid to a part between saidfirst and second valves such that said exhaust device allows said liquidto expel air from said part between said first and second valves,wherein a pump is in a mid-section of said feed pipe, with said firstand second strainers being on an upstream side of said pump, whereinassociated with said one of said first and second strainers is (i) athird valve on an upstream side of said one of said first and secondstrainers, (ii) a fourth valve on a downstream side of said one of saidfirst and second strainers, and (iii) another exhaust device, andwherein a branch pipe connects said part between said first and secondvalves and a part between said third and fourth valves with a part ofsaid feed pipe on a downstream side of said pump, such that (i) feedingsaid liquid to said part between said first and second valves comprisesfeeding said liquid, via said branch pipe, to said part between saidfirst and second valves, and (ii) said another exhaust device is forexhausting air from a part between said third and fourth valves inresponse to said liquid when introduced from said branch pipe to saidpart between said third and fourth valves and expelling air from saidpart between said third and fourth valves.
 2. The method according toclaim 1, wherein feeding said liquid, via said branch pipe, to said partbetween said first and second valves comprises feeding said liquid, whendischarged from said pump, via said branch pipe to said part betweensaid first and second valves such that said liquid replaces the airexpelled from said part between said first and second valves.
 3. Amethod for handling an easily polymerizable substance, comprising:feeding liquid containing an easily polymerizable polymer selected fromthe group consisting of acrylic acid, methacrylic acid, and estersthereof, from a device for distilling said easily polymerizable polymerunder reduced pressure, via a feed pipe, having an upstream end thatopens in said device for distilling, through one of a first strainer anda second strainer, with said first and second strainers being connectedin parallel with one another to said feed pipe; measuring a differencebetween a first pressure in said feed pipe on an upstream side of saidone of said first and second strainers and a second pressure in saidfeed pipe on a downstream side of said one of said first and secondstrainers; and based on the difference between said first and secondpressures, as measured, (i) stopping the feeding of said liquid throughsaid one of said first and second strainers, and (ii) initiating feedingof said liquid, via said feed pipe, through the other of said first andsecond strainers, wherein associated with the other of said first andsecond strainers is (i) a first valve on an upstream side of the otherof said first and second strainers, (ii) a second valve on a downstreamside of the other of said first and second strainers, and (iii) a firstexhaust device for exhausting air from a part between said first andsecond valves, wherein associated with said one of said first and secondstrainers is (i) a third valve on an upstream side of said one of saidfirst and second strainers, (ii) a fourth valve on a downstream side ofsaid one of said first and second strainers, and (iii) a second exhaustdevice for exhausting air from a part between said third and fourthvalves, wherein a pump is in a mid-section of said feed pipe, with saidfirst and second strainers being on an upstream side of said pump, andwherein a branch pipe connects said part between said first and secondvalves and said part between said third and fourth valves with a part ofsaid feed pipe on a downstream side of said pump.
 4. The methodaccording to claim 3, wherein initiating feeding of said liquid throughthe other of said first and second strainers comprises (i) using saidfirst exhaust device to exhaust air from said part between said firstand second valves, and (ii) feeding said liquid, when discharged fromsaid pump, via said branch pipe to said part between said first andsecond valves so as to replace the air exhausted from said part betweensaid first and second valves.
 5. A method for handling an easilypolymerizable substance, comprising: feeding liquid containing an easilypolymerizable substance from a decompressor, used to handle said liquidunder reduced pressure, via a feed pipe, having an upstream end thatopens in said decompressor, through one of a first strainer and a secondstrainer, with said first and second strainers being connected inparallel with one another to said feed pipe; measuring a differencebetween a first pressure in said feed pipe on an upstream side of saidone of said first and second strainers and a second pressure in saidfeed pipe on a downstream side of said one of said first and secondstrainers; and based on the difference between said first and secondpressures, as measured, (i) stopping the feeding of said liquid throughsaid one of said first and second strainers, and (ii) initiating feedingof said liquid, via said feed pipe, through the other of said first andsecond strainers, wherein associated with the other of said first andsecond strainers is (i) a first valve on an upstream side of the otherof said first and second strainers, (ii) a second valve on a downstreamside of the other of said first and second strainers, and (iii) a firstexhaust device for exhausting air from a part between said first andsecond valves, wherein associated with said one of said first and secondstrainers is (i) a third valve on an upstream side of said one of saidfirst and second strainers, (ii) a fourth valve on a downstream side ofsaid one of said first and second strainers, and (iii) a second exhaustdevice for exhausting air from a part between said third and fourthvalves, wherein a pump is in a mid-section of said feed pipe, with saidfirst and second strainers being on an upstream side of said pump, andwherein a branch pipe connects said part between said first and secondvalves and said part between said third and fourth valves with a part ofsaid feed pipe on a downstream side of said pump.
 6. The methodaccording to claim 5, wherein initiating feeding of said liquid throughthe other of said first and second strainers comprises (i) using saidfirst exhaust device to exhaust air from said part between said firstand second valves, and (ii) feeding said liquid, when discharged fromsaid pump, via said branch pipe to said part between said first andsecond valves so as to replace the air exhausted from said part betweensaid first and second valves.